VoIP (Voice over Internet Protocol) is one of the popular communication technology. In VoIP, SIP (Session Initiation Protocol) defined by IETF is the most widely used protocol because of its simple structure, expandability and easy operation.
Referring to FIG. 1, it shows a conventional telephone system. International station 1 connects with domestic station 2, domestic station 2 connects with A station 3, B station 4 . . . etc. A station 3 connects with a switchboard 31 of a university, a switchboard 32 of an elementary school . . . etc. The switchboard 31 has a phone number 0227712171, and manages some conventional extensions 2178. The switchboard 32 has a phone number 0227710846, and manages some conventional extensions 6000.
A conventional extension 2178 cannot dial directly to a conventional extension 6000. The conventional extension 2178 has to dial the phone number 0227710846 of the switchboard 32, after hearing a voice guidance, then dial the number “6000” for being connected with the conventional extension 6000.